The Caroline note is an interesting tip in determining this young man's identity.
My theories:
1.) Carolines were friends, possibly roommates at college
2.) The lack of an area code seems to indicate that perhaps Jason knew one or both of the girls lived in the same area, or that he was familiar enough with the area where they came from that they didn't feel a need to include an area code (ex: they all went to UNC in Chapel Hill, just put apartment phone numberetc.)
3.) The concert was in late June, and most colleges and universities had let out in Mid-May. I don't think this was a dorm phone as the Carolines would have moved out for the summer already by the time Jason ran into them at the concert.
I also think that perhaps Jason may have met up with the girls and hung out, maybe slept over at their house/apt. and the girls left a note when they left early in AM next day etc. I wouldn't leave a note for a friend unless they were sleeping in the morning after a concert or a party, assuming they would call me later OR if I had waited around at a meeting point and that person never shower-I'd leave a note like this on my front door, car windshield etc. Maybe the girls stayed at Jason's place that night, perhaps they were all from DC or NOVA area and snuck out early in the morning the next day leaving the note so as not to wake him. Another idea is that the Carolines were tailgating with him before show, went to show, and in the area before cells and texting, left a note at a prearranged site (maybe a third party car they had been hanging out around) to indicate they had left show already and to get in touch later?
ALSO as a side note...Grateful Dead tickets, a Dead shirt from the tour, FILA shoes, perfect teeth-- indicates to me that Jason was probably from an upper/upper middle class socio-economic group. I also think the name Caroline is often associated with girls that would have Jason's peer age who would come from a similar socio-economic background. The fact that he didn't have a wallet or any other luggage seems to suggest to me that other items he was traveling with did not find their way to him after the concert. Most teens and 20 somethings would have had at least some cash, maybe a velcro wallet with Learner's Permit, license etc. inside. I find it strange that a kid who appears to have clothing that indicates perhaps a middle/upper class socio-economic background has NOTHING on them.